Weather

Weather

TDC

Kevin Cameron

ALL OUR MOTORING ACTIVITIES OCCUR within the highly variable atmosphere. Our engines breathe the air, and are, directly or indirectly, cooled by it. Therefore, local changes in air temperature, barometric pressure, and moisture content can change how engines run.

Rider Rich Schlachter and I crossed one of the Colorado passes at near

12,000 feet. Having been cooped up in the van for too many hours, we jumped out and sprinted towards a nearby park. After only fifty feet, we wheezed to a stop, chests heaving. There was no air. Engines have the same problem. Power depends upon the weight of correct fuel-air mixture burned per second. At

12.000 feet, the air pressure is only 65 percent of what it is at sea level, so maximum power is also only 65 percent of sea level power. If your engine has carburetors, the problem is worse. Carburetors don't measure absolute pressure; they only compare venturi pressure with outside pressure. Therefore, for equal throttle opening, they deliver essentially the same amount of fuel at 12,000 feet as at sea level. The result is that the fuel-air mixture becomes tremendously rich (too much fuel in proportion to air) at high altitude; power is reduced in the first place because of reduced air density. It is reduced even more because every combustion cycle is cooled by all that extra fuel.

Power is at a maximum at a 12.5:1 air-fuel ratio, so race engine tuners change carburetor jetting as local atmospheric pressure changes, whether the change results from weather or from the altitude of the race track. Standard atmospheric pressure at sea level is 29.92 inches of mercury. To ensure that the engine receives the desired fuel-air mixture strength even if local air pressure rises or falls, use simple proportion. Divide the local air pressure by the standard pressure (29.92"), and multiply that number times the main-jet number that we've found to work best at standard pressure (the numbers on motorcycle main-jets are proportional to flow capacity). Thus, if the barometric pressure goes down, we jet down, and vice versa.

Most fuel-injection systems include a device that continuously measures absolute local air pressure, and does both the math and the re-jetting for us as we go. Thus, the power loss at altitude with fuel injection is in simple proportion to air pressure, but there is no enrichment loss as there would be with carburetors.

Air also loses density as its temperature rises, so engines pump less weight of air the hotter it gets. Since there is now less oxygen to combine with fuel, power falls. As the weather cools, the air becomes more dense, and the engine pumps a greater weight of air, and so needs more fuel to go with it. Novice racers sometimes discover this relationship the hard way. As afternoon Daytona practice ends, the air temperature is pleasantly warm, but next morning it may be downright chilly. If the bike runs hard in morning practice without re-jetting, it may now get more air than before, but the same amount of fuel. It will therefore be lean (too much air in relation to fuel). If it was correctly jetted for maximum power in yesterday afternoon's warm conditions, it will certainly lose power now from being lean, and may also knock and overheat.

Compensating for temperature is not quite so simple because zero degrees on the thermometer does not represent the point at which gases cease to gain density as they cool. Therefore, we must transform the temperature scale to one referenced to absolute zero, the temperature at which all molecular motion reaches minimum. For example, absolute zero on the Fahrenheit scale is 460 degrees below the zero on our familiar thermometers. If the air temperature was 85 degrees F at the end of yesterday afternoon's practice session, that is 460+85=545 degrees absolute. If this morning it is 62 degrees F, that is 460 +62=522 degrees absolute. To find out how much richer we have to jet, we divide 545 by 522 to get 1.044. If yesterday our machine was close to the limit, running really sharp on a 210 main-jet, this morning’s jet ought to be 21 Ox 1.044=219.3, so we'll put in 220 jets.

Because it’s easy to make mistakes in arithmetic when rushing to get ready for the next practice, most tuners use an instrument that does both the measurements and the figuring, translating changes in barometric pressure and temperature into a single number, proportional to air density. This is an air-density gauge, which relates local air density to the density at standard temperature and pressure (STP) conditions. If the meter reads 1.02, it means local air density is 2 percent higher than at STP. If you have previously figured your engine's needs at STP. you now have the information you need to jet approximately for today’s weather; you put in jets that flow 2 percent more fuel than the jets you'd use at SI P conditions.

Sometimes even this turns out wrong. Last spring at the Loudon, New Hampshire, national, experienced 250 rider/tuner Rich Oliver noticed his engine was still acting rich even on the “correct" jetting. New England is horribly humid in the spring, and the excess of water molecules in the air was taking up room normally occupied by oxygen. Oliver’s engine was therefore getting too little combustion air. and needed jetting leaner than what was “correct" according to the meter. The effect is small, but when you're near the limit, it can be important.

With street motorcycles, all these peak-power niceties are essentially irrelevant. A compromise jetting is adequate for the range of air conditions usually encountered. The exception is the case in which a machine jetted for sea level is then ridden mostly in or around some high-altitude place like Den ver or Mexico City ( or v i e e versa). The owner's manual or an experienced dealer can provide re-jetting information.